Non-aqueous formulations for ram and screen extrusion-spheronisation

The use of non-aqueous cellulose-based formulations for extrusion-spheronisation (E-S) is investigated. A 10 wt% hydroxypropyl cellulose/isopropyl alcohol solution (HPC/IPA) was identified as a suitable sticky liquid binder for preparing non-aqueous pastes. Preliminary tests were performed on a series of pastes using a ram as well as a laboratory roller screen extruder, since the former is commonly used in batch testing and the latter replicates the shear range in a manufacturing screen extruder. Pellets with acceptable size and shape distributions were obtained with Avicel® HFE-102 NF/HPC/IPA for ram E-S, and with Avicel® RC-591/HPC/IPA for screen E-S. Further investigation was performed with calcium carbonate added as a model active pharmaceutical ingredient. Both formulations were able to generate pellets with acceptable size and shape characteristics at up to 50 wt% carbonate loading: further work is required to optimise yields.

A comparison of screen and ram extrusion-spheronisation of simple pharmaceutical pastes based on microcrystalline cellulose

The performance of two laboratory-scale extrusion apparatuses used to approximate the action of an industrial screen extruder, namely a multi-holed die ram extruder and a roller screen extruder, were compared. Both devices featured short dies (ram 2mm, screen 1mm) with die diameter 1mm and hole area fraction approaching 0.25. A series of water/microcrystalline cellulose (MCC) pastes with water contents varying from 45 to 60 wt% were extruded and pellets obtained from subsequent spheronisation of the extrudates characterised in terms of size and shape. Each device exhibited a different range of processing windows for acceptable spheronised products, with the ram apparatus being able to extrude a wider range of paste water contents than the screen device. The pellets obtained from extrusion-spheronisation (E-S) of the pastes via the screen device were in general smaller, with a wider size distribution, than those from ram E-S. These results are attributed to the different mechanical histories experienced by the pastes in the two types of extruder, which lead to different extrudate densities being achieved. MCC/water/calcium carbonate pastes were also tested, where the latter component represented a 'hard' (non-deformable) active pharmaceutical ingredient. Addition of calcium carbonate increased the stiffness of the paste, which could be countered by adjusting the water content of the deformable MCC/water matrix within the extrudability limits of the latter material.

Effects of drying technique on extrusion–spheronisation granules and tablet properties

Extrusion-spheronisation was used to generate smooth, highly spherical granules of a microcrystalline cellulose/propyl gallate/water paste. Freeze-drying retained the shape and size of the granules, whereas oven-drying produced roughened granules due to the uneven shrinkage of the wet powders. Compaction of one size fraction indicated that the granule strength differed noticeably, with the oven-dried samples producing tablets of lower voidage for a given applied compaction pressure. There was a reasonable correlation between tablet crushing strength and voidage. Major differences were observed in tablet dissolution, with the freeze-dried material exhibiting a two-regime behaviour and an initial dissolution rate constant an order of magnitude greater than the oven-dried form. Both the voidage and dissolution characteristics are postulated to be determined by the microstructure established during drying.

Liquid phase migration in the extrusion and squeezing of microcrystalline cellulose pastes

Extensive movement of the liquid phase relative to the solids in solid-liquid pastes during extrusion forming is an undesirable process phenomenon. The impact of formulation and flow pattern on liquid phase migration (LPM) during extrusion of model pharmaceutical pastes (40-50 wt% microcrystalline cellulose/water) has been investigated by ram extrusion through square-entry and 45 degrees conical-entry dies, and by lubricated squeeze flow (extensional flow). Threshold velocities for LPM were observed in both configurations. Squeeze flow testing showed that dilation during extension can cause LPM, while ram extrusion featured both dilation effects and drainage due to compaction. The threshold velocities observed in the two configurations agreed when presented as characteristic shear rates. The threshold velocity increased with paste solids content.

The production of homogeneous extrudates of microcrystalline cellulose pastes

The homogeneity of water-based microcrystalline cellulose (MCC) paste extrudates was investigated during ram extrusion as a function of ram velocity. Variations in the water content of the extrudates were caused by liquid phase migration within the paste. The evolution in water content was measured by sectioning and drying the extrudate, and the subsequent homogeneity was quantified by the standard error in water content. The homogeneity of the extrudates was found to decrease as the ram velocity decreased. This result was also inferred from the rate of increase of the extrusion pressure. The extrudate homogeneity was significantly improved by compensating for water migration in the barrel during the compaction stage. This was achieved using a non-uniform initial paste billet, created by packing the barrel with layers of paste of different water contents. This technique also produced a smaller variation in extrusion pressure over the ram displacement range, and a reduction in water loss from the upstream paste compact into the extrudate and/or through the apparatus tooling.

Effects of liquid phase migration on extrusion of microcrystalline cellulose pastes

The behaviour of water-based microcrystalline cellulose pastes undergoing ram extrusion has been investigated. Factors affecting the redistribution of water within the extruding paste and the upstream barrel compact, such as the initial water content, extrusion rate and die geometry, have been considered. The rates of dewatering for these given systems were characterised by the gradients of the extrusion pressure-ram displacement profiles. A linear relationship between the pressure-displacement gradient and the inverse square root of the paste velocity was obtained for a given paste and extrusion geometry. At velocities where water migration was significant, the extrudate was found to have a higher water content than that of the paste in the barrel at any given time; both the extrudate and the barrel paste decreased in water content with increasing ram displacement. Spheronisation of extrudate samples has shown that the redistribution of liquid during extrusion is an important factor affecting the quality of the spheres. A paste flow model, incorporating pseudo-plastic and shear deformation terms, was used to predict the change in extrusion pressures caused by liquid phase migration. The model parameters were obtained as functions of water content and gave good agreement with the experimental extrusion profiles.